This invention relates to transducers, and in particular, to transducers having a full-bridge sensing structure and an amplifier circuit connected to the sensing structure.
Transducers, in particular, transducers incorporating a full-bridge (also called a "wheatstone" bridge) sensing structure are used in a variety of areas including automotive and industrial applications to measure pressures, temperatures and other parameters. In automobiles for example, pressure sensors monitor, among other things, oil pressure and transmission fluid pressure.
It is known to include in such a transducer a full-bridge sensing structure to generate a differential voltage output in response to a stimulus and an amplifier circuit connected to the sensing structure to amplify the differential voltage output. Using the pressure sensor example from above, the sensing structure senses a pressure within a system and generates a signal functionally related to that pressure. The signal is then amplified and provided as the output of the transducer.
It is also known in the art to calibrate the output of an integrated circuit transducer through the process of laser trimming of resistors in the circuit. Laser trimming is achieved by making laser cuts in the polysilicon resistor material to increase the resistance of the resistor in the circuit and thereby modifying in a precise way the characteristics of the circuit.
In one known construction, the sensing structure and the amplifier circuitry are provided as discrete or separate integrated circuit components. However this construction substantially increases the cost of manufacturing the sensor because both the sensing bridge resistors and the amplifier biasing resistors require laser trimming or calibration prior to connecting the circuits together to form the complete transducer circuit. Moreover, once the sensing bridge and the amplifier circuit are electrically connected, testing and additional trimming or calibration is required to assure that the two circuits are appropriately matched and that the sensor is functioning properly.
One attempt to solve the above-identified problems led to the development of a fully integrated pressure sensor as shown in the Motorola Semiconductor Technical Data Sheet illustrating a Motorola "MPX5050" Pressure Sensor. However, to insure accuracy of the pressure sensor output signal, the amplifier circuit requires a plurality of amplifiers connected in the instrumentation mode and includes at least seven integrated resistors requiring costly laser trimming.